1. Field of Invention
The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to an electric coolant pump control strategy for an HEV to maintain vehicle powertrain and accessory component temperatures within optimal operating temperatures.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles powered by an internal combustion engines (ICE) is well known. Vehicles powered by electric motors attempt to address these needs. However, electric vehicles have limited range and limited power capabilities and need substantial time to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called Hybrid Electric Vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 (Severinsky).
The HEV is described in a variety of configurations. Many HEV patents disclose systems in which an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a Series Hybrid Electric Vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A Parallel Hybrid Electrical Vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that together provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A Parallel/Series Hybrid Electric Vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is typically known as a xe2x80x9cpowersplitxe2x80x9d configuration. In the PSHEV, the ICE is mechanically coupled to two electric motors in a planetary gearset transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque powers the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery if a regenerative braking system is used.
The desirability of combining an ICE with an electric motor is clear. The ICE""s fuel consumption and emissions are reduced with no appreciable loss of vehicle performance or range. Nevertheless, there remains a substantial opportunity to develop ways to optimize HEV operation.
One area of development is maintaining the desired operating temperature of the HEV components. A cooling system maintains optimal component operation and performance. Overheated components adversely affect efficiency and may eventually cause component failure.
A typical prior art cooling system for an ICE vehicle has a coolant fluid in an enclosed loop that passes through certain vehicle components and a heat exchanger (radiator). A heater core is also typically added to vent engine heat into the passenger compartment as desired. The engine and transmission components typically require cooling from a liquid cooling system. As the coolant circulates through these components in the closed loop, it absorbs heat that is released as the coolant passes through the radiator and heater core.
Coolant flow in the prior art cooling system is typically controlled by a pump driven front-end accessory drive (FEAD). As engine speed increases, the speed of the pump also increases allowing more coolant flow through the system. Additionally, a thermostat within the loop only allows coolant flow through the radiator after the coolant temperature reaches a level at which the engine temperature has stabilized and is considered xe2x80x9cwarmed up.xe2x80x9d
Though simple and reliable, the prior art coolant control system comprising a pump and a thermostat is inadequate for HEVs. For example, the HEV has additional components that require cooling. Further, the prior art coolant pump does not function when the engine is off. Thus, the typical vehicle accessories driven by the FEAD (including the coolant pump, air conditioning, and power steering) in a conventional vehicle must be powered by an alternate source in the HEV to maintain their functionality when the engine is not running.
Electric coolant pump control systems exist in the prior art for electric motor powered vehicles, but those systems do not completely meet the coolant control system needs of an HEV. In Barrie, et. al. (U.S. Pat. No. 5,217,085) (Ford Motor Company) a coolant system control module varies the coolant system pump speed by varying its duty cycle in response to a temperature sensor. The electric pump delivers fluid to a temperature responsive valve that controls flow to the heat exchanger. The Barrie patent does not address the cooling needs of the vehicle""s internal combustion engine. Thus, a second prior art coolant system for the engine is also required. Unfortunately, two coolant systems within the same vehicle increase the complexity and cost of the HEV.
Accordingly, the present invention provides a method and system to control the liquid cooling needs of all HEV powertrain and accessory components in a single loop system. The method and system implement an electric coolant pump control strategy that sets a pump duty cycle (and thus coolant flow rate) according to a calibratable table as a function of engine temperature and electric motor temperature.